1. Field of the Invention
This invention relates to a soft and absorbent paper web useful in tissue, toweling, sanitary and like products and to methods for its manufacture.
2. Description of the Prior Art
Disposable paper articles such as tissues, towels, sanitary and like products made from bulky, absorbent paper webs are familiar articles of commerce. In the conventional manufacture of such products, it is customary to use paper webs which, during the manufacturing process, have been subjected to one or more pressing operations over the entire surface of the paper web, as laid down on the Fourdrinier wire or other forming surface, prior to final drying. In the conventional process, the pressing operations involve subjecting a moist paper web supported on a papermaking felt to pressure developed by opposing mechanical members such as rolls. While this operation expels water from the web thereby reducing the drying load, smoothing the surface of the web, and increasing its tensile strength, a paper of relatively high density and relatively stiff character is produced.
Various techniques have been suggested as improvements to the conventional papermaking process so that softer, bulkier and more absorbent paper webs can be made. One of the most significant such improvements is that described and claimed by Sanford and Sisson in U.S. Pat. No. 3,301,746 which issued Jan. 31, 1967. These inventors discovered that an improved, bulky, soft, absorbent paper web can be produced if the wet paper web is thermally predried without substantial compression to a fiber consistency substantially in excess of that normally found in webs entering the final thermal drying section of a conventional tissue papermaking process. The thermally predried web is imprinted with a knuckle pattern by a conveying and imprinting fabric, and finally dried without disturbing and imprinted knuckle pattern. The end result of this process, which is hereinafter referred to as the Sanford and Sisson process, is a sheet of tissue paper having lower apparent density and greater bulk than that which may be produced on a conventional paper machine while, at the same time, exhibiting adequate tensile strength for commercial uses of such paper.
Products made by either the conventional or the Sanford and Sisson process have been made using a variety of conventional wood pulps. The most common wood pulp used is that generally referred to as chemical pulp and is well known to those skilled in the papermaking art. This pulp consists essentially of delignified, relatively long, flexible fibers. Specific examples of these pulps are the well known kraft and sulfite pulps. While articles such as tissues, towels, and sanitary products made from conventional chemical pulps have found wide application among the consuming public, and while those made by the Sanford and Sisson process have received very favorable reception, two disadvantages are inherent in the use of conventional chemical pulps. First, there is the inability of manufacturers to further increase the absorbency, bulk, and softness of their products because of the inherent limitation of significantly decreased paper web tensile strength at desirable low densities. And second, there is the disadvantage associated with the inherent waste in chemical pulping operations. Conventional chemical pulping processes such as the well known kraft and sulfite processes yield only about 50 percent of the input wood as pulp and, concurrently, result in waste streams that either pollute the environment or are difficult and expensive to process so as to avoid pollution.
Two approaches have been used in an attempt to rectify the second disadvantage mentioned supra. The first of these is the use of conventional mechanical pulp in papermaking operations. (Conventional mechanical pulp is sometimes referred to as groundwood or stone ground pulp.) In this particular pulping operation, sections of the whole tree are comminuted in specially designed grinding machines. This process results in more than 90 percent of the input wood being emitted as pulp suitable for papermaking. This high yield, and absence of chemicals, substantially reduces the inherent waste and pollution problems associated with chemical pulping operations. Unfortunately, the conventional mechanical pulping operation results in pulp which is composed of relatively short fibers, highly damaged fibers, and large quantities of fiber and ligneous debris. Papers made from this pulp by any papermaking process are generally quite dense and stiff and are therefore unsuitable for use in consumer articles such as tissues, towels, sanitary and like products.
The second approach is a relatively recent improvement in mechanical pulping known as thermomechanical pulping. (This process is sometimes referred to as the pressure refining of wood fibers and as the Asplund process.) In this process, soaked wood chips are subjected to mechanical abrasion at temperatures in excess of the boiling point of water. It is postulated that the lignin binding the wood fibers is softened or plasticized by the elevated temperatures and the dissociation of the fibers is thereby facilitated. Pulps prepared by a normal thermomechanical pulping process are characterized by the relative freedom from damage of the individual fibers, the greater or lesser coating of the individual fibers with lignin, and the generally unmodified length of the fibers. As in conventional mechanical pulping processes, more than 90 percent of the wood entering the process is emitted as pulp suitable for papermaking and the potential for pollution caused by the process is greatly reduced as compared to normal chemical pulping methods.
Whereas conventional mechanically refined pulp has been found to be undesirable for use in absorbent, soft, bulky consumer articles such as tissues, towels, sanitary and like products, thermomechanically refined pulp can be used in conventional papermaking processes to produce such products. When thermomechanical pulp is used in conventional tissue papermaking processes to make low density paper webs, the relationship between web dry tensile strength and web density is the same as that relationship for conventional chemical pulps. That is to say, with conventional chemical pulps the decrease in tensile strength which occurs with decrease in web bulk density follows essentially a straight line relationship; substitution of thermomechanical pulp into the paper web, either as a portion of the fiber furnish or as the total fiber furnish, can be described by the same tensile-density relationship. The net result of the substitution of thermomechanical pulp into tissue, towel, sanitary and like products made by conventional papermaking processes and like products made by conventional papermaking processes is a improvement in the overall economy of such products and a reduction in the total pollution potential associated with manufacturing such products, but there is little resulting practical improvement in such products. (That is to say, there is little practical increase in bulk, softness, or absorbency because any decrease in density is accompanied by a corresponding decrease in tensile strength which makes the resulting products impractical to use.) The consumer products obtained using thermomechanical pulp in conventional papermaking processes are essentially identical to those which can be made with conventional chemical pulp.